The present disclosure relates to a gas turbine engine and, more particularly, to a blade tip clearance control system therefor.
Gas turbine engines, such as those that power modern commercial and military aircraft, generally include a compressor section to pressurize an airflow, a combustor section to burn a hydrocarbon fuel in the presence of the pressurized air, and a turbine section to extract energy from the resultant combustion gases. The compressor and turbine sections include rotatable blade and stationary vane arrays. Within an engine case structure, the radial outermost tips of each blade array are positioned in close proximity to a shroud assembly. Blade outer air seals (BOAS) supported by the shroud assembly are located adjacent to the blade tips such that a radial tip clearance is defined therebetween.
When in operation, the thermal environment in the engine varies and may cause thermal expansion and contraction such that the radial tip clearance varies. The radial tip clearance may be influenced by mechanical loading, e.g., radial expansion of the blades and/or their supporting disks due to speed-dependent centrifugal loading, and thermal expansion, e.g., of the blades/disks on the one hand and the non-rotating structure on the other. The radial tip clearance is typically designed so that the blade tips do not rub against the BOAS under high power operations when the blade disk and blades expand as a result of thermal expansion and centrifugal loads. When engine power is reduced, the radial tip clearance increases. The leakage of core air between the tip of the turbine blades and the BOAS may have a negative effect on engine performance, efficiency, fuel burn, and component life.
To facilitate increased engine performance, at least some engines include a blade tip clearance control system to maintain a close radial tip clearance. One type of blade tip clearance control system is an Advanced Passive Clearance Control (APCC) system that utilizes a control ring of low thermal expansion material and a series of segmented BOAS carriers, typically 12-14 per stage, of higher thermal expansion material. The APCC facilitates a reduced blade tip gap without tip rubbing and improved fuel consumption performance. Although effective, the series of segmented BOAS carriers may result in reduced stability at the segmented carrier intersurfaces that could potentially result in non-circular support of the BOAS and blade tip rub conditions.